The combustion of solid fuels in grate, fluidized bed and rotary kiln firing systems is generally carried out in two stages. The combustion of solid matter initially takes place in the first stage, accompanied by a mostly hypostoichiometric introduction of primary air. In this context, the solid fuel passes through the individual steps of drying, degasification of the volatile constituents, as well as burning-off of the fixed carbon.
The high calorific value gases produced during the combustion of solid matter are mixed in the second combustion stage, which is accompanied by the hyperstoichiometric introduction of secondary air at high temperatures and, as a result, are fully combusted.
Hydrochloric acid (HCl) and sulfur oxides (SO2 and SO3) are produced by the combustion of chlorine- and sulfur-containing fuels, such as household refuse or biomass (for example, wood and straw). The SO3 content in proportion to SO2 is mostly very low; relative to the total concentration of sulfur oxides (SO2+SO3), the SO3 content is mostly less than 5%. Household refuse and, in particular, hazardous wastes can contain other halogen compounds, such as bromine and iodine compounds, in addition to chlorine compounds. Bromine and iodine compounds behave similarly to the chlorine compounds during combustion and cause similar problems.
The alkali metals (potassium and sodium) and other metals contained in the fuel partially produce chlorides during combustion of the solid matter. Under the high temperatures prevailing in the combustion bed, alkali and metal chlorides have a relatively high vapor pressure, so that considerable amounts thereof are released in the gaseous state into the waste gas.
The thermal energy of the hot combustion and wastes gases from an incineration is mostly utilized for heating a steam boiler. When heat is released, the combustion and waste gases are cooled, the saturation vapor pressure of the volatile inorganic compounds (alkali and metal chlorides) being fallen short of. As a result, these chloride compounds condense and/or resublimate and, together with the inert and carbon particles contained in the waste gas, produce a chloride-containing fly ash. This partially settles on the heat exchanger surfaces of the steam boiler and forms an undesirable coating. The deposited fly ash is essentially made up of complex mixtures of silicates, sulfates, oxides, carbonates and chlorides.
Generally, the chloride content in these ash deposits undesirably promotes the formation of chlorinated organic compounds, such as dioxins (PCDD/F), for example, and additionally causes considerable corrosion damage to the metallic components, particularly of the boiler. In this case, the alkali chlorides (NaCl and KCl) play a critical role due to their high concentration in the fuel, the flue gas and in the ash deposits. The formation of PCDD/F, as well as the corrosion of the metallic boiler material are both attributable to the formation of chlorine (Cl2) within and underneath the fly ash deposits on the boiler surface. In addition, within and underneath these chloride-containing ash deposits on the boiler surface, chloride (Cl2) is formed from the hydrochloric acid (HCl) contained in the waste gas by what is generally referred to as the Deacon process, due to a catalytic action of metal oxides/metal chlorides (in particular, Cu and Fe) contained in the fly ash.
Direct reactions of alkali chlorides with the mostly iron-containing boiler material likewise induce a significant Cl2 formation. The Cl2 generated brings about, in turn, an oxychlorination of the particulate carbon (soot particles) present in the ash deposits. This PCDD/F formation path, known as de novo synthesis, plays an absolutely critical role in the waste incineration process for the PCDD/F present in the raw gas. By avoiding the formation of Cl2, the PCDD/F formation can be effectively suppressed, so that the need for expensive waste-gas purification processes for reducing PCDD/F can be substantially eliminated.
The Cl2 formed within and underneath the chloride-containing boiler deposits has a very corrosive effect on metallic and, in particular, iron-containing boiler materials. What is generally referred to as the chlorine-induced boiler corrosion increases greatly with a rising wall temperature of the boiler made of metallic materials. Increased boiler corrosion is associated with considerable costs which, in turn, significantly limits the range of steam parameters (temperature T and pressure p) in the steam generation, particularly in the waste incineration process. Consequently, boilers of waste and biomass incineration plants are mostly operated at only relatively low steam qualities of T=400° C., p=40 bar, which also greatly limits the thereby attainable efficiency when generating electrical energy by steam turbines.
As a function of the waste gas composition and the prevailing combustion temperatures in and downstream of the waste gas burnout zone following the burnout of solid matter, alkali hydroxides may be formed from the alkali chlorides released from the combustion bed in accordance with the following reaction equations (1) and (2).KCl+H2O→KOH+HCl  (1)NaCl+H2O→NaOH+HCl  (2)
In and downstream of the waste gas burnout zone, the alkali metals (potassium, sodium) may be present as chlorides and/or hydroxides. In this context, a portion of the alkali hydroxides react further in the high-temperature range in and downstream of the waste gas burnout zone in the oxidizing atmosphere, both with the SO2 contained in the waste gas, as well as with HCl, to form chlorides and sulfates in accordance with the following reaction equations:2KOH+SO2+½O2→K2SO4+H2O  (3)2NaOH+SO2+½O2→Na2SO4+H2O  (4)KOH+HCl→KCl+H2O  (5)NaOH+HCl→NaCl+H2O  (6)
The formation of alkali chlorides and/or sulfates in and downstream of the waste gas burnout zone depends in this context on the ratio of SO2/HCl concentrations and on the local process conditions (temperature and cooling rate of the waste gas).
To avoid the aforementioned undesired chemical reactions that lead to the formation of chloride-containing substances and the unwanted effects resulting therefrom, efforts are directed to increasing the SO2 concentrations in the combustion gas. A method for reducing the dioxin formation in combustion processes by increasing the SO2 concentration in the flue gas was described for the first time in 1986 by Griffin (Griffin R. D.: A new theory of dioxin formation in municipal solid waste combustion; Chemosphere, vol. 15, issue 9-12 (1986) pages 1987-1990). It was theorized therein that Cl2 is reduced by reaction with SO2 as a result of the subsequent homogeneous gas phase reaction.Cl2+SO2+H2O→SO3+2HCl  (7)
In known combustion systems, the SO2 concentration is increased by adding sulfur-containing fuels, sulfur or sulfur compounds to the combustion. Recent investigations show the sulfation of the fly ash, and thus the reduction of the chloride content of this fly ash and of the fly ash deposits, is the critical reaction for reducing or avoiding the formation of Cl2. Particularly at high temperatures, the chlorides undergo sulfation at a high reaction rate by the SO2 contained in the flue gas, whereby sulfates are formed and HCl is released in accordance with the following equations.2NaCl+SO2+½O2+H2O→Na2SO4+2HCl  (8)2KCl+SO2+½O2+H2O→K2SO4+2HCl  (9)
U.S. Pat. No. 4,793,270 describes introducing CS2, CaS and SO2 into the incineration process to reduce the dioxin formation rate in the course of a waste incineration.
DE 199 53 418 A1 describes adding amidosulfuric acid and sulfonamide to the fuel to reduce dioxins in the waste gas of chemical processes.
To reduce corrosion, DE 198 49 022 A1 describes introducing sulfur-containing chemicals, such as SO2 and MgSO4, into the combustion gas.
DE 602 11 476 T2 (from WO 02/059526) describes adding a sulfur-containing chemical, such as (NH4)2SO4, NH4HSO4, H2SO4 or FeSO4, to reduce corrosion.
DE 101 31 464 B4 describes a method for the low-corrosion and low-emission co-incineration of highly halogenated wastes in waste incineration plants which provides for adding sulfur or sulfur-containing chemicals.
DE 198 02 274 C2 describes a method for reducing corrosion during operation of a boiler of a waste incineration plant, where sulfur or sulfur-containing compounds are introduced into the combustion chamber or the hot waste gases before reaching the corrosion-prone heating surfaces.
WO 06/124772 A2 and WO 06/134227 A1 describe adding Fe(SO4)3, Al2(SO4)3, and/or SO2, SO3, H2SO4, sulfur or sulfur salts to reduce corrosion in steam boilers.
In principle, the co-incineration of sulfur, sulfur compounds or sulfur-containing fuels (such as municipal sewage sludge, discarded waste tires or sulfur-containing coal) or the charging of SO2/SO3, H2SO4 or other sulfur-containing compounds, for example (NH4)2SO4, into the waste gas takes place before entry into the steam boiler.
DE 103 38 752 B9 describes a process-integrated SO2 cycle in the course of a waste incineration that does not require any external charging of sulfur or sulfur compounds.
All methods are based on the fact that the sulfation, and thus the reduction of the chloride content of the fly ash and boiler ash deposits, is achieved with increasing SO2 and/or SO3 concentration in the flue gas. Generally, it is disadvantageous that, in existing incineration plants, relatively high SO2 and/or SO3 concentrations in the waste gas are required in proportion to the HCl and to the compounds to be sulfated, such as alkali, alkaline-earth and metal compounds.
EP 0 193 205 B1 describes a circulating fluidized-bed combustion in which sulfur-containing fuels are burned while alkaline sorbents (CaO) are added to the fluidized bed to separate sulfur compounds. The dwell time of the combustion gases in the primary combustion zone is 1-3 s (650-1095° C.) and, in the waste gas burnout zone, 0.2-2 s. A special sulfide/sulfate solids oxidation zone is configured as a dense-phase fluidized bed in the solids return line of the cyclone leading to the fluidized bed. The alkali sulfide contained in the separated solids is oxidized by the introduction of air in this oxidation fluidized bed into sulfate at waste gas temperatures ranging from 590-985° C. and solid residence times from 1-30 s.
WO 1982/04036 describes a method for recovering fluorine from the carbonaceous material from the linings and/or cathodes of reduction cells, where the fluorine is liberated as gaseous hydrogen fluoride by heating the carbonaceous material in the presence of oxygen, water and sulfur dioxide. The reaction time is approximately one hour.
WO 1989/05340 describes a carbonaceous fuel composition. During combustion accompanied by the addition of Ca and Mg compounds, as well as of an oxidation catalyst, a reduction in sulfur oxide and nitrogen oxide emission is achieved.